Thermoplastic elastomers are known. They have many of the properties of thermoset elastomers, yet they are processable as thermoplastics. One type of thermoplastic elastomer is a thermoplastic vulcanizate, which may be characterized by finely-divided rubber particles dispersed within a plastic. These rubber particles are crosslinked to promote elasticity. Thermoplastic vulcanizates are typically prepared by dynamically vulcanizing a rubber contained in a blend that includes the rubber and a thermoplastic resin.
Some thermoplastic vulcanizates are commercially produced by dynamically curing the rubber with a peroxide curative. While this curing process has the potential to produce technological useful thermoplastic vulcanizates, the use of peroxide curatives can be problematic. In particular, peroxide curatives can degrade the thermoplastic resin, which results in the reduction of mechanical properties.
This problem is further aggravated where harder thermoplastic vulcanizates are desired. As is known in the art, thermoplastic vulcanizates may be advantageously produced in a variety of hardnesses. The hardness can be adjusted by the level of thermoplastic resin that is included in the blend. Unfortunately, as the amount of thermoplastic resin is increased, the amount of peroxide curative per part of rubber that is required to effect dynamic vulcanization must also be increased even though the proportionate amount of rubber is decreased. As the amount of peroxide curative is increased, the degree of thermoplastic resin degradation is likewise increased.
Attempts have been made to overcome the problem. For example, U.S. Pat. No. 4,985,502 teaches the use of less peroxide curative. Unfortunately, the use of less curative typically hinders the ability to fully cure the rubber, which results in a sacrifice in engineering properties. Also, U.S. Pat. No. 5,656,693 attempts to alleviate the problem of thermoplastic resin degradation by employing a rubber terpolymer (e.g., ethylene-α-olefin-diene terpolymer, which may be referred to as an EADM rubber) that includes vinyl norbornene polymeric units. These rubbers are move efficiently curable with peroxides and therefore the amount of peroxide required to achieve full cure is reduced; this results in reduced impact on the thermoplastic resin.
Other attempts to alleviate problems caused by the use of peroxide curatives include the use of highly crystalline EADMs. As is generally known in the art, EADM rubbers that include a high percentage of ethylene (i.e., greater than 75 or 80 mole percent) are characterized by ethylene crystallinity. These EADM rubbers are readily curable by peroxide curatives. As a result, their use in the manufacture of thermoplastic vulcanizates have potential benefit inasmuch as less peroxide may be needed to cure the rubber.
The use of these crystalline EADM rubber, however, limits the breadth of the thermoplastic vulcanizates that can be produced. For example, the amount of oil added to thermoplastic vulcanizates is deleteriously limited. As those skilled in the art appreciate, the addition of more than 50-70 parts by weight oil to thermoplastic vulcanizates prepared from highly crystalline EADMs is extremely problematic. These thermoplastic vulcanizates that employ crystalline EADMs in larger amounts of oil will tend to exhibit exudation and stickiness of parts fabricated therefrom. U.S. Pat. No. 6,610,786 sets forth numerous examples that employ highly crystalline EADMs in the manufacture of thermoplastic vulcanizates that have little or no oil. This problem apparently exists despite the fact that this patent produces thermoplastic vulcanizates by a process where additional rubber or thermoplastic resin is added after dynamic vulcanization; the second-step addition of the thermoplastic resin may occur within the same extruder used to dynamically vulcanize the original product or within a second extruder.
Other multiple-stage processes for the production of thermoplastic vulcanizates are likewise known as disclosed in U.S. Pat. No. 6,288,171. Thermoplastic vulcanizates have been produced by “let down” processes whereby soft (e.g., Shore A of about 50) thermoplastic vulcanizates are produced and pelletized. After pelletization, the pellets are blended with polypropylene pellets and the blend is melt mixed and extruded. This process suffers from processing inefficiencies and is believed to impact performance properties of the thermoplastic vulcanizates. In another example, solid polypropylene is added in a down stream barrel during extruder production of the thermoplastic vulcanizate by using a crammer feeder. But, the solid polypropylene is not readily miscible with the molten thermoplastic vulcanizate in the time scale of this process and therefore processing and performance shortcomings are encountered.
Because thermoplastic vulcanizates are technologically important compositions, there is a continued need to develop improved thermoplastic vulcanizates that have an overall balance of improved properties. And, because peroxide-cured thermoplastic vulcanizates have potential to contribute to this technology, there remains a need to improve peroxide-cured thermoplastic vulcanizates and processes for making the same.